1. Field of the Invention
The present invention relates to a scintillation camera wherein a radioisotope (hereinafter called as "RI") is labeled by a medicine having a property concentrating on a specific internal organ and the medicine is supplied to a human body to be examined, and .gamma.-rays emitted from RI at random are detected for a fixed period of time, thereby obtaining a distribution image of RI, i.e., a scintigram.
2. Description of the Related Art
A scintillation camera produces an image of the radioisotope distribution by detecting individual gamma rays or photons passing through a collimator and causing scintillation events in a scintillator in a detector. The scintillation caused by the gamma rays are provided to photomultipliers arrayed in the detector. Each photomultipliers provides a electrical signal dependent on the location of scintillation. Calculation circuit provides orthogonal (XY) coordinate signals and energy signal proportional to the energy level for each scintillation event from the signal outputs of photomultiplier. Compensating or calibration circuit compensates XY coordinate signals for non-linearity and energy signal for non-uniformity which the detector has. Window or pulse height analyzer circuit analyzes if the energy level of Z signal is within a predetermined energy window or width, so that only XYZ signals whose Z signal is within the predetermined energy width contribute to an image. Image memory accumulates a count at locations corresponding to XY signals, thereby, an image is obtained.
In a SPECT (Single Photon Emission Computed Tomography), further detector rotates around a human body to obtain radioisotope distributions of it from many directions. A conventional reconstruct technique reconstructs a tomogram image of radioisotope distribution from such radioisotope distributions obtained from many directions.
Gamma rays and photons cause Compton-scattering in the human body, collimator or scintillator. Detector detects both non-scattered and scattered gamma rays or photons. A conventional scintillation camera cannot remove the scattered gamma rays or photons from non-scattered ones sufficiently, in particular whose energy level is the same as the non-scattered. The scattered gamma rays or photons makes an image blurred.
K. F. Koral et al. "SPECT Compton-scattering correction by analysis of energy spectra, "J.Nucl.Med., vol. 29, pp. 195-202, 1988 discloses that scattered gamma rays or photons are estimated by energy spectrum obtained using a narrow energy window changeable along energy axis. However, in the method, it takes a long time to obtain energy spectrums for an image region and it is impossible to acquire energy spectrums for an image region at same time.
A multi-nuclide examination in which a plurality of energy-different RIs labeled by medicine having a different property are simultaneously provided to the object to be examined is useful for the quantitative examination of the function of the organ. In the multi-nuclide examination, the Z signal must be separated for each other. In conventional, the separation processing is performed by a plurality of different windows. Such a separation method is useful for only the time when the photoelectric peak between the nuclides is clearly separated on an energy axis. Therefore, such a separation method cannot be used in a case that the photoelectric peak between the nuclides is overlapped on the energy axis.
The above-mentioned energy spectrum includes much information, which is useful for the diagnosis. However, in the conventional scintillation camera, only a part of such information is used.